The present invention generally concerns value documents.
In this specification the term value documents, as the term implies, are documents which involve a value of some nature, for example bank notes, credit cards, passes, or tickets.
One form of value document, as is to be found for example in U.S. Pat. No. 4,684,795, carries, on at least one of its surfaces, a security feature or element which on the one hard includes a magnetic layer comprising a dispersion of magnetisable particles in a binder and on the other hand a security layer which has an optical-diffraction effect, for example a hologram or a computer-generated diffraction layer, an interference layer or a diffraction grating. In that document the security layer is superimposed on the magnetic layer at least in a region-wise manner thereon while the surface of the security layer which is towards the magnetic layer has a spatial structure with an optical-diffraction effect. The security layer is further provided with a reflective non-magnetisable metal layer.
The present invention also concerns embossing foils and more particularly but not exclusively hot embossing foils as may preferably be used for the production of value documents of the kind indicated above. Such an embossing foil may typically comprise a backing film and a transfer layer which is releasable therefrom and which, starting from the backing film, comprises at least one security layer in the form of a transparent lacquer layer with an optical-diffraction effect and which includes a spatial structure with an optical-diffraction effect at its surface remote from the backing film, a reflective metal layer which is disposed on the spatial structure of the security layer, a magnetic layer comprising a dispersion of magnetisable particles in a binder, and a bonding or adhesive layer which serves for fixing the transfer layer to a substrate and which is possibly formed by the magnetic layer or is combined therewith. Above-mentioned U.S. Pat. No. 4,684,795 also described the principles of producing such a value document by means of an embossing foil, while a detailed description of suitable embossing foils with a magnetic layer and a security layer having an optical-diffraction effect is to be found in DE 34 22 910 Cl.
Hitherto, the production of value documents or hot embossing foils of that kind generally involved a procedure in which the reflective metal layer which serves to clearly show the structure with the optical-diffraction effect and which is applied to the correspondingly patterned surface of the security layer is formed by aluminium or aluminium alloy applied by vapor deposition in a vacuum. However under some circumstances the use of aluminium for the reflective metal layer can give rise to major problems, in particular when the value document in question is to be used in a humid atmosphere. More particularly, it has been found in some circumstances in such a situation that the aluminium layer is damaged or destroyed at at least some spots thereon or changes at least in regard to its appearance, for example by becoming discolored. That can result in the proper functioning of the security feature being adversely affected. That becomes apparent in particular when the structure of the security layer, which has an optical-diffraction effect, is a machine-readable structure, for example a hologram or a computer-generated diffraction structure.
Investigations have shown that the destruction of or damage to the aluminium layer on the document is probably to be attributed to the fact that the magnetisable particles of the magnetic layer, which in fact are usually iron oxides in different degrees of oxidation, react with the aluminium of the reflective metal layer, triggering off corrosion of the aluminium. The precise mechanism involved in that reaction is not known. It is suspected that the damage is to be attributed to the fact that the iron oxide pigments which are used as the magnetisable particles act as proton donors, another factor being the consideration that the iron oxide pigments used have pH-values in a range of between 3.0 and 5.5. In that case, under some circumstances, local elements can be formed between the magnetisable particles on the one hand and the aluminium serving as the reflective metal layer on the other hand, with corresponding damage to the aluminium layer.